Accurate knowledge of the environment such as shape, color, distance and arrangement of various objects in the surroundings can be useful in robotic applications, aerial or underwater vehicles or autonomous driving. Of the multiple sensing options in such applications, light detection and ranging devices (LiDARs) have certain advantages, e.g., resolution, range and precision. A LiDAR can estimate the distance to or geometric features of surroundings on a point by point basis to form a point cloud of reachable or visible objects. To measure points in the cloud, individual pulses of light can be projected onto various objects and the associated time of flight (time between the transmission of light and the detection of the reflection) can be estimated. To cover the area of interest in the environment and form a point cloud, pulses of light can be issued repeatedly in various directions. The orientation of the projected laser and the measured time of flight (TOF) can be used to assign a spatial location to every observed point in the 3-dimensional space. In a similar fashion power of the emitted light can be modulated and the phase lag between the detected and emitted light can be used instead of TOF for distance estimation.
In many applications including autonomous driving, increasing the point cloud density can be very helpful in various tasks such as mapping, localization and perception. However, the point cloud density in state of the art LiDARs such as in [1, 2] is often very low due to a limited number of laser sources and, similarly, a limited number of detectors. Given the space and cost constraints there is often a limited number of laser diodes and detectors that can be accommodated in a LiDAR system. The necessity to include a multitude of sensors and light sources is in part due to optical design and signal detection requirements. Eye safety, energy consumption and also operating temperature requirements limit acceptable laser power. Low laser power, limited detection dynamic range and short duration of laser pulses combined with often very weak diffusive reflection off the majority of surfaces make detection of reflected light very challenging. This challenge is even greater in outdoor applications where background lighting can undermine detectability of reflected light. As such, it is very important to capture as much light as possible to increase the detectability of the reflected light. This is often done by using relatively large lenses. However, large lenses feature large focal distance and hence have a very limited field of view. As such, to accommodate an acceptable field of view, e.g., 20-30 degrees, one needs to use multiple sensors placed far apart in the lens focal plane. A common and effective design is to assign a laser diode with proper orientation to each and every detector. Another reason justifying the application of multiple laser sources is rooted in the limitations of laser pulsing rate. In state of the art LiDAR systems each laser diode is capable of generating 10 to 20 thousand pulses per second at relatively high laser power due to the associated temperature rise. Such a pulsing rate is not sufficient for building dense point clouds at a reasonable frame/sweep rate relevant to applications such as autonomous driving or high speed robotics, etc. As such, application of multiple laser sources would help to increase the pulsing rate beyond that provided by a single light source. The requirement for unambiguous detection of reflected light further limits the feasible pulsing rate. For a single pair of light source and detector, issuance of new pulses should be delayed until the reflection(s) associated with the latest pulse is (are) measured. Otherwise it would be unclear as to which reflection corresponds to which of the emitted past light pulses. As an example for a 300 m detection range the consecutive pulses should be at least 2 μs apart for unambiguous estimation of time-of-flight (TOF). By assigning a single laser source to every detector and applying a multitude of such pairs this limitation can also be side-stepped. Although application of multiple laser sources and multiple sensors is feasible, it brings about severe cost and resolution constraints and limits the capabilities of LiDARs.
The present invention addresses these limitations of the prior art by applying an array of scanning mirrors. This invention further uses modulation of wavelength or intensity of projected light to enable unambiguous estimation of TOF or phase lag for dense depth map formation using a minimal number of light sources and detectors all fitted in a small package.